1. Field of the Invention:
This invention relates to photographic optical systems with a light arrangement to the light meter, and more particularly to photographic optical apparatus with a light arrangement suited to single-lens reflex cameras and electronic still cameras using image pickup tubes or solid state image pickup elements such as a CCD.
2. Description of the Prior Art:
As the single-lens reflex type of the camera using silver-halide film had the highest potential for expanding the system, it has developed to a high degree of versatility. Its typical optical system is partly shown in FIG. 1.
In the same figure, a photographic lens 100 is followed by a flippable total reflection mirror 101 in front of a focal plane shutter 102 and a film plane 103. A focusing screen 104, a pentagonal roof type prism 105, and an eyepiece 106 constitute a finder optical system through which an eye 107 can observe an image on the focusing screen 104 in the conjugate plane to the film plane 103 with respect to the photographic lens 100.
To measure the object brightness with the light entering through the photographic lens 100, the so-called TTL type light meter has its sensor, in most cases, positioned either on the bottom face or the exit face of the pentagonal roof type prism 105.
The silver-halide film can be replaced by an image pickup tube or a solid state image pickup element such as a so-called CCD to thereby form an electronic camera. For such an electronic camera, particularly that using the single-tube type or the monoplane type, image pickup, it is necessary to employ additional optical members such as an infrared ray cut filter, an optical low pass filter, a color separation filter and a protection glass plate arranged in a space between the photographic lens 100 and the image plane.
The infrared ray cut filter is used for preventing color purity from being lowered with the reproduced image becoming turbid, since the image pickup board or element is sensitive to light of the infrared region which the human eye cannot see. The optical low pass filter rids the object image of a space frequency component which is higher than a certain level and which turns to spurious signals, for example, when the single-tube type camera employs the stripe filter for color separation purposes and is usually in the form of a crystal plate to utilize the phenomenon of double refraction. The color separation filter, when in a frequency separation system or a phase separation system, is striped or mosaic in structure so that the image is separated in three basic colors, namely, red, green and blue. The protection glass covers the image pickup surface and, in some cases, also serves as a filter.
In the case of the electronic still camera already in existence, the optical low pass filter is put in front of the quick return mirror for reflecting light from the object lens to the finder. However, this makes the finder image difficult to see. To avoid this, therefore, a crystal plate must be used in the optical path of the finder to cancel that effect. This measure results in a reduction in flexibility of design of the interior element arrangement of the camera and also is a source of increased production cost.